It's possible Opus thinks Explorer Replacement is enabled on the machine due to some registry values which didn't get cleaned up when the USB version exited (e.g. it could happen if power is cut to the machine when the USB version is running in Explorer Replacement mode).
If you look at HKEY_CLASSES_ROOT\Folder\shell using RegEdit, what's the b[/b] value set to, if anything? If it's "openindopus", exit Opus and wait a few seconds, then delete the value and it should make things work again.
(Alternatively, there's a .reg file in How to manually remove Explorer Replacement settings which will do the same and tidy up a couple of other things. Be sure to exit Opus and wait a few seconds before double-clicking it.)